(1) Field of the Invention
Subject invention is related to interfering noise elimination in an electrical circuit and more particularly to a tracking notch filter.
(2) Description of the Prior Art
When low signal level electronic measurements are made, interference from harmonically related interfering signals such as the AC power line and its harmonics is often a problem. Techniques such as shielding and proper grounding are employed to reduce the interference. Once the signals are recorded, such as on a tape recorder, any interference remaining is also recorded. The person analyzing the recorded data must therefore use other means to remove interfering harmonics from the playback signals. Two methods currently used are digital adaptive filtering and analog notch filtering.
Digital adaptive filters locate, track and attenuate time stable harmonics. They effectively attenuate the power line harmonics but have the undesirable effect of indiscriminately reducing the signals of interest with any other stable frequencies.
High Q narrow bandwidth analog notch filters attenuate a very narrow predetermined frequency band per filter and thus do not affect the signals of interest. However, high Q analog circuits display a high sensitivity to component tolerance making the narrow notches unstable and difficult to tune. Since a separate filter must be employed for each power line harmonic, each such filter requires separate tuning. Furthermore, tape recorders add wow and flutter which shift the frequencies slightly, in turn reducing the effectiveness of such fixed frequency analog filters. There is thus a need for a filter which automatically tracks and eliminates interfering harmonics of a power line without distorting the desired signals.